In paging communication systems considerable difficulty is encountered because of "shadowing." A paging signal is normally a simplex signal transmitted by a paging system transmitter at a uhf or microwave frequency. Since, at the frequencies involved, signal propagation is essentially line-of-sight with limited penetration, paging signal reception is often impeded or prohibited in the signal shadows caused by buildings, tunnels, hills, and other artificial and natural topographical features. For example, in an exemplary satellite communication system, paging signals are limited by a 27 dB link margin. This link margin is only adequate for limited building penetration. A pager within a medium-sized or larger building may have to be close to an outside wall in order to receive a page.
In traditional communications, the standard solution to the general problem of reception in a shadowed area is to provide a repeater. A repeater, in elementary form, is a receiver connected to a transmitter. Through an unshadowed receiving antenna, a repeater receives the original signal then retransmits it through a transmitting antenna positioned to cover the shadowed area. The retransmitted signal is typically shifted in time or frequency from the original signal, thereby increasing spectrum requirements in the vicinity of the repeater.
A paging repeater would be a repeater specifically engineered to provide this function for a paging communication system. Thus, a pager in an otherwise shadowed location might be able to receive a signal from a paging repeater when an adequate signal could not be received directly from a paging system transmitter.
A satellite-based paging communication system that utilizes low-earth-orbit (LEO) satellites exacerbates the spectrum-increase problem. A given paging signal received from a such a satellite may be extensively shifted in frequency relative to the satellite's transmission frequency due to the Doppler effect. In an exemplary paging system transmitting in the L-band, the Doppler shift may be in excess of .+-.37 KHz. When receiving an exemplary paging signal only a little over 40 KHz wide, a center-frequency shift of .+-.37 KHz may prevent a pager from obtaining an adequate lock on the signal, impeding reception.
Were a conventional repeater to be used to repeat a paging signal containing a significant Doppler shift, retransmission of that signal would represent a significant waste of bandwidth and further complicate the problems inherent in the reception of such a signal. For example, around 240 KHz of spectrum would be required to convey a 40 KHz paging signal through a repeater.
Accordingly, a need exists for a paging system that incorporates paging repeaters on a non-interfering basis so as to compensate for paging signal shadowing wherever required without passing on unnecessary Doppler shifts in the signals being repeated. Simultaneously, the proper reception of Doppler-shifted paging signals from low-earth-orbit satellite transmitters should be allowed.